This has been bugging me for quite some time now.
Ever since upgrading GTK+ to the 2.6.x series, the gtk based programs on my computer would crash when doing something like browsing the open-file dialog or opening files on the desktop.
When running the programs from command line, the error would look like this (swap meld with any other gtk program):
(meld:17337): GLib-GObject-CRITICAL **: g_object_ref: assertion `G_IS_OBJECT (object)' failed (meld:17337): Pango-CRITICAL **: _pango_engine_shape_covers: assertion `PANGO_IS_FONT (font)' failed (meld:17337): GLib-GObject-CRITICAL **: g_object_ref: assertion `G_IS_OBJECT (object)' failed (meld:17337): Pango-CRITICAL **: _pango_engine_shape_shape: assertion `PANGO_IS_FONT (font)' failed Pango-ERROR **: file shape.c: line 75 (pango_shape): assertion failed: (glyphs->num_glyphs > 0) aborting...
With the GTK+2.4.x series it worked. So I suspected either that or Pango, but reinstalling both did not help.
And until recently the gentoo forums did not have an appropriate solution either.
But today I looked again and finally found a solution:
Change the Application and Desktop font of Gnome from ClearlyU to Bitstream Vera Sans !